Ballistics Recognition and Identification Systems

Types of Toolmarks

Firearms examiners deal with the striated toolmarks that gun barrels impart to fired bullets and with the impression and striated marks that various parts of firearms impart to cartridge cases (for example, breechblock, ejector, extractor, and firing pin marks) and other ammunition components. Striated toolmarks are patterns of scratches or striae that result from the parallel motion of ammunition components against firearm components. Impression toolmarks result from the perpendicular, pressurized impact of firearm components on ammunition components.

Impression and striated toolmarks have class, subclass, and individual characteristics. The distinctively designed features of types of guns are reflected in class characteristics. For example, the rifling impressions on bullets are class characteristics that reflect the number, width, and direction of [Page 552]twist of the lands and grooves in the types of barrels that fired them.

Subclass characteristics, which are present in only some toolmarks, arise when manufacturing processes create batches of tools, such as firearm components, with similarities in appearance, size, or surface finish that set them apart from other tools of the same type. The toolmarks produced by tools in the batch have matching microscopic characteristics, called subclass characteristics, that distinguish them from toolmarks produced by other tools of the type.

Firearms identification is premised on the existence of individual characteristics that are unique to the toolmarks each individual tool produces and that correspond to random imperfections or irregularities on tool surfaces produced by the manufacturing process or subsequent use, corrosion, or damage. If the same class characteristics are found on evidence and test toolmarks (for example, the same rifling impressions on a test fired bullet and an evidence bullet recovered from a crime scene), a firearms examiner uses a comparison microscope to compare the toolmarks' individual characteristics (for example, microscopic striations within rifling impressions). The object is to determine whether the individual characteristics are so similar that one and the same tool (for example, a particular gun barrel) must have produced both the test and the evidence toolmark.

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